Early detectors used oil-diffusion secondary pumps, and their analysis units were protected by means of cryogenic traps which condensed the oil vapor from the diffusion pump. However, the test pieces were not protected from the oil vapor backscattered by mechanical primary pumps having oil seals and serving to pre-evacuate the test pieces to pressures of about 10.sup.-2 mbars.
In the following generation of leak detectors, the oil-diffusion secondary pumps were replaced by mechanical-type pumps (e.g. turbomolecular blade pumps or Holweck-type pumps etc.) which, by construction, do not contain any lubricant (pumps having magnetic bearings) or contain so little lubricant (pumps having ball bearings) that they do not contaminate the analysis unit or the test piece. However, the test piece is still subjected to the action of vapor from the lubricant in the oil seal of the mechanical primary pump during the pre-evacuation stage, i.e. the stage during which the test piece and the pre-evacuation primary pump are directly connected together, even though, by using mechanical secondary pumps capable of operating at pressures of about one mbar, the pre-evacuation stage can be caused to take place under viscous flow only, thereby limiting backscattering of oil vapor towards the test piece. However, that remains unsatisfactory for ultra-clean applications in which the test piece may suffer from the slightest trace of hydrocarbon (the main ingredient of lubricants for mechanical primary pumps having oil seals).
Furthermore, whenever primary pumps with oil seals are used, the surrounding environment receives oil vapor or mist, in particular during high-pressure operation. This can be eliminated by connecting the outlet of the pump to a suction system, but that involves connecting additional pipes leading from the detector to the ventilation system.
Another drawback of leak detectors using primary pumps with oil seals results from the fact that certain precautions must be taken when transporting the apparatus and during operation thereof:
the apparatus must be horizontal both during transport and during operation; and PA1 the pump must be emptied before it is transported in a non-pressurized aircraft or under conditions in which it is subjected to large amounts of vibration. PA1 diaphragm pumps; PA1 dry vane pumps; PA1 rotary piston pumps; PA1 reciprocating piston pumps; PA1 peristaltic pumps; and PA1 screw pumps; etc. PA1 they have suction limit pressures that are very high relative to the lower delivery pressures of the secondary pumps; PA1 they have compression ratios that are relatively low for lightweight gases such as helium, and this increases the recovery time of the detector once it has absorbed a certain quantity of helium, and may even make it impossible to use the apparatus for detecting leakage rates that are very low; and PA1 they have pumping speeds that are very low at low suction pressures of less then 10 mbars.
In order to avoid the drawbacks arising from the fact that a fluid is used for lubricating and sealing such primary pumps, they can be replaced by "dry" pumps:
However dry pumps suffer from several drawbacks:
An object of the present invention is to provide a leak detector which uses a dry pump as the primary pump, while mitigating the above-mentioned drawbacks and automatically providing good pumping speed at low pressures of less than 10 mbars for pumping out the enclosure to be tested.